Chain of Responsibility

Intent

Avoid coupling the sender of a request to its receiver by giving more
than one object a chance to handle the request. Chain the receiving
objects and pass the request along the chain until an object handles
it. [GoF, p223]

Launch-and-leave requests with a single processing pipeline that
contains many possible handlers.

An object-oriented linked list with recursive traversal.

Problem

There is a potentially variable number of "handler" or "processing element"
or "node" objects, and a
stream of requests that must be handled. Need to efficiently process
the requests without hard-wiring handler relationships and precedence,
or request-to-handler mappings.

Discussion

Encapsulate the processing elements inside a "pipeline" abstraction; and
have clients "launch and leave" their requests at the entrance to the
pipeline.

The pattern chains the receiving objects together, and then passes any
request messages from object to object until it reaches an object
capable of handling the message. The number and type of handler
objects isn't known a priori, they can be configured dynamically. The
chaining mechanism uses recursive composition to allow an unlimited
number of handlers to be linked.

Chain of Responsibility simplifies object interconnections. Instead of
senders and receivers maintaining references to all candidate
receivers, each sender keeps a single reference to the head of the
chain, and each receiver keeps a single reference to its immediate
successor in the chain.

Make sure there exists a "safety net" to "catch" any requests which
go unhandled.

Do not use Chain of Responsibility when each request is only handled by
one handler, or, when the client object knows which service object
should handle the request.

Structure

The derived classes know how to satisfy Client requests.
If the "current" object is not available or sufficient, then it
delegates to the base class, which delegates to the "next" object,
and the circle of life continues.

Multiple handlers could contribute to the handling of each request.
The request can be passed down the entire length of the chain, with
the last link being careful not to delegate to a "null next".

Example

The Chain of Responsibility pattern avoids coupling the sender of a
request to the receiver by giving more than one object a chance to
handle the request. Mechanical coin sorting banks use the Chain of
Responsibility. Rather than having a separate slot for each coin
denomination coupled with a receptacle for the denomination, a single
slot is used. When the coin is dropped, the coin is routed to the
appropriate receptacle by the mechanical mechanisms within the bank.
[Michael Duell, "Non-software examples of software design patterns",
Object Magazine, Jul 97, p54]

Check list

The base class maintains a "next" pointer.

Each derived class implements its contribution for handling the
request.

If the request needs to be "passed on", then the derived class
"calls back" to the base class, which delegates to the "next" pointer.

The client (or some third party) creates and links the chain (which
may include a link from the last node to the root node).

The client "launches and leaves" each request with the root of the
chain.

Rules of thumb

Chain of Responsibility, Command, Mediator, and Observer, address how
you can decouple senders and receivers, but with different trade-offs.
Chain of Responsibility passes a sender request along a chain of
potential receivers.

Chain of Responsibility can use Command to represent requests as
objects. [GoF, p349]

Chain of Responsibility is often applied in conjunction with
Composite. There, a component's parent can act as its successor.
[GoF, p232]